1. Field of the Invention
The present invention relates to an occupant restraint system in which an airbag is disposed in a folded state along an upper edge of a door opening of a vehicle body and the airbag is inflated by a gas generated by an inflator when the vehicle is involved in a side collision or a rollover so that it is deployed into a curtain shape along an inner side face of an occupant compartment.
2. Description of the Related Art
A conventional occupant restraint system is known in, for example, Japanese Patent Application Laid-open Nos. 2000-33847 and 11-235965. The airbag of this type of occupant restraint system includes an inflatable part that inflates when a gas is supplied thereto and is deployed along the inner side face of an occupant compartment to restrain an occupant.
The airbag of the conventional occupant restraint system does not include a vent hole, whereby the airbag inflates to become hard when a gas is supplied from an inflator, thus making it difficult to gently restrain the occupant. Further, there is a possibility that the occupant might suffer an increased peak acceleration and a rebound phenomenon, that is, a phenomenon in which the occupant rebounds from the airbag.
Although providing a vent hole in the airbag so as to release part of the gas therein is conceivable to prevent an excessively increased internal pressure within the airbag, since the vent hole makes it impossible to retain the gas within the airbag, there is a possibility that the airbag might excessively deflate due to a load restraining the occupant, thus shortening the time available for restraining the occupant with the airbag.
Furthermore, the airbag of the conventional occupant restraint system is deployed both in a vehicle side collision and a vehicle rollover to protect the head of the occupant, but since a strong impact is momentarily exerted when the vehicle is involved in the side collision, in order to suppress the peak acceleration exerted on the occupant, it is desirable that excess gas within the airbag is efficiently discharged through a vent hole, thus lowering the internal pressure of the airbag. On the other hand, when the vehicle rolls over, it is necessary for the airbag to be maintained in a deployed state for a comparatively long period of time, but if a vent hole is provided in the airbag, there is a possibility that the gas might leak through the vent hole and the airbag deflate too soon, resulting in an inadequate performance of restraining the occupant. If the vent hole is made small or eliminated in order to prevent the early deflation of the airbag, there is then a possibility that the internal pressure of the airbag might excessively increase when the vehicle is involved in a side collision.